1. Field of the Invention
The present invention relates generally to electrical testing apparatus, and more particularly relates to apparatus for electrically testing printed circuit boards.
2. Description of Related Art
The simultaneous electrical testing of the circuitry on the opposite sides of a printed circuit board is conventionally carried out by contacting a multiplicity of test contact points on opposite sides of the board with first ends of electrically conductive, longitudinally resilient test pin members commonly referred to as "pogo" pins. The nonresilient opposite end of each test pin is electrically connected to an individual wire which is operatively connected at its opposite end to a test circuit that receives and combinatively analyzes the multiplicity of electrical signals flowing from the circuit board test contact points to the test circuit via the pogo pins and their associated wires.
This conventional method of electrically testing a printed circuit board has a variety of well known problems, limitations and disadvantages. For example, a modern high density printed circuit board may easily have, on its top and bottom sides, several thousand individual test points which must be electrically connected to the automatic test and analysis system. The time required to effect the point-to-point wiring from the corresponding number of pogo pins to the test and analysis circuit is accordingly quite high, and may very easily result in wiring errors. Additionally, there are inherent variances in the length and placement of the individual wires in such conventional individually wired test fixtures. This often causes integration problems where multiple fixtures are attached to multiple test systems in the manufacturing environment.
Conventional point-to-point wiring of a multiplicity of closely adjacent pogo pins to the associated test and analysis system quite often leads to undesirable electrical "coupling" of the individual signals that travel through the also closely adjacent individual wires, thereby adversely affecting the electrical performance of the overall test fixture apparatus. Another problem relating to the use of individual wires for each test pin is that the wires define relatively long paths through which the individual electrical signals must travel to and from the printed circuit board under test. These long electrical signal paths often adversely affect the ability to perform electrical testing of the printed circuit board at high frequencies.
A variety of mechanical problems are also typically associated with conventional test fixture apparatus employing point-to-point wiring between the multiplicity of test pins and associated contact points in the automatic test and analysis circuit. For example, it is common practice to drive the printed circuit board under test into engagement with the transversely oriented test pin members using a vacuum force imposed directly on the printed circuit board. As a practical matter, this requires that the circuit board under test be nonpermeable, and essentially precludes the use of vacuum-driven actuation systems in the electrical testing of circuit boards of perforated or highly porous construction. Additionally, the inherent flexing of the circuit board under test due to atmospheric pressure on its top surface, as well as the opposing forces applied on the underside of the circuit board by the necessary spring-loaded test pins, in many cases renders the use of a vacuum-driven actuation system, to effect the fixture-to-unit under test interface, impractical. This is particularly true when a relatively large area printed circuit board is to be tested.
In an effort to eliminate the various problems associated with point-to-point wiring in the electrical testing of printed circuit boards, various proposals have been made for effecting the necessary interface between the circuit board being tested and the test and analysis circuitry without using individual wires. For example, under one previous proposal the electric signal transfer between the test contact points on the opposite sides of the circuit board and the test and analysis circuit is effected by using interface boards positioned above and below the circuit board under test.
A large number of openings are formed through the interface boards, with the wire connection ends of conventional test pins being inserted and soldered into some of the openings, and connector members being inserted and soldered into the remaining openings in each interface board and engaging conductive contact points on the outer sides of the boards. Electrically conductive etch lines are formed on the inner side surfaces of the interface boards to appropriately interconnect their test pins and connector members, and the inwardly projecting depressible ends of the test pins are aligned with the top and bottom side test contact points on the circuit board being tested for engagement therewith. Suitable circuitry is added to electrically interconnect the top and bottom interface boards to permit the necessary interconnection thereof to the test and analysis circuitry.
While this testing scheme succeeds in eliminating much of the point-to-point wiring used in more conventional test fixture apparatus, it has two primary disadvantages which render it less than entirely satisfactory as a replacement for test fixtures using the conventional point-to-point wiring format. First, the construction of the interface boards is a laborious and time-consuming task since each of the multiplicity of test pins, and their associated connector members, must be inserted and soldered into the through-holes formed in their associated interface board. Additionally, for each of the interface boards, the test pins must be appropriately interconnected to their associated connector members by means of the aforementioned electrically conductive etch lines.
Second, this through-hole connection technique, particularly when a "high density" printed circuit board is to be tested, requires that a large surface area of each interface board be dedicated to the openings required for the connection of the test pins and connector members thereto. Accordingly, the board surface area available for the necessary etch paths is correspondingly reduced. Due to the typically great number of individual etch paths required, this in turn requires that the interface boards have surface areas considerably larger than the opposite surface areas of the printed circuit board being tested. Because of this heretofore unavoidable requirement, it is often the case that the circuit board being tested cannot be positioned directly over the upwardly projecting probes of the test and analysis circuit. Instead, the circuit board being tested must be horizontally shifted relative to the probe points of the test and analysis circuitry, thereby substantially and quite undesirably increasing the overall horizontal dimensions of the test fixture apparatus.
From the foregoing it can be readily seen that it would be highly desirable to provide an electrical testing system for printed circuit boards which eliminates or at least substantially reduces the problems, limitations and disadvantages heretofore associated with conventional test systems of the general types described above. It is accordingly an object of the present invention to provide such a system.